Frequency Selective Surfaces Enable Mems Gas Sensor
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FREQUENCY SELECTIVE SURFACES ENABLE MEMS GAS SENSOR Irina Puscasua, Martin U. Prallea, Mark P. McNeala, Nicholas Moeldersa, Lisa Lasta, William Hoa, Anton C. Greenwalda, James T. Dalya, Edward A. Johnson*a Ihab El-Kadyb, Rana Biswasb a Ion Optics, Inc., 411 Waverley Oaks Road, Waltham, MA 02452 b Ames Laboratory and Department of Physics, Iowa State University, Ames, IA 50011 ABSTRACT We have developed a thermally stimulated narrow-band infrared source for sensing, spectroscopy and thermophotovoltaic applications by combining the unique advantages of two different structures: a photonic crystal that consists of an array of holes etched into a dielectric substrate and a periodically perforated metallic thin film. The dielectric photonic crystal structure is passive and exhibits a strong absorption at resonance. This acts as a radiation reservoir for the conductive array, which plays an active role through plasmon interactions and is opaque at all wavelengths except those at which coupling occurs. We have fabricated the arrays on silicon, silicon dioxide and silicon nitride substrates using MEMS-based processing methods. Infrared spectroscopic studies were used to characterize reflection, absorption and emission in the 2 to 14 micron range showing narrow band resonance. Spectral tuning was accomplished by controlling symmetry and lattice spacing of the arrays. The effects of the etch depth, metal and dielectric properties have been studied experimentally and theoretically. These structures have been used as an emitter/detector sensor chip to selectively detect industrial pollutants like carbon dioxide.
INTRODUCTION The combination of integrated circuit thin-film technologies and silicon micromachining has facilitated the development and improvement of many thermally based sensors, such as gas sensors, flow sensors and pressure sensors as well as thermal radiation emitters. 1-5 In this paper we present a novel thermally stimulated mid-infrared source that emits radiation within a narrow range of wavelengths, similar to frequency selective surfaces or photonic crystals. The emission wavelengths are defined by a metal coated silicon-air photonic crystal etched into the emitter surface. This allows for tunability of the radiation and narrow emission at a designed wavelength. This technology promises a new class of tunable infrared emitters with high power in a narrow spectral band that are critical for sensing, spectroscopy, and thermophotovoltaic applications. We are going to discuss in this paper the influence of different parameters on the spectral response. Transfer matrix calculations simulate well the position and the strengths of the absorption features and give theoretical insight into the physical mechanisms possible. *
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The emitter consists of a passive dielectric substrate that exhibits at the surface an array of indentation
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